The present invention relates to electric traction motor vehicles and, more particularly, to a system for matching the torque developed by a direct current electric traction motor in a time ratio controlled power system to that of a similar motor being operated in a cam controlled power system.
Transit vehicles such as subway cars are typically operated in trains, i.e., a plurality of cars are mechanically and electrically coupled to operate as a single unit. However, each car in the train has its own propulsion and control system, the control system being connected to receive operating command signals from a lead car in the train. Historically, the propulsion and control systems in each car have been essentially identical electrical systems, e.g., direct current (d-c) series wound traction motors provided motive power and a cam controller regulated the tractive effort produced by the d-c motors. Since each car's electrical motive power system was essentially identical, an operator's command to a train of cars would result in each car responding essentially identically. Thus each car's motive power system need only be capable of supplying the power required for that car. This maximum level of required power becomes a fundamental consideration since, in order to make a transit or subway car economically practical, passenger space is maximized at the expense of available space for motive power equipment. As will be appreciated, if the motive power system on one of the cars of a train attempts to develop more acceleration than that of other cars of the train, that car will attempt to pull more than its share of the weight of the train to the detriment of its motive power system.
In a cam controlled direct current traction motor power system, a plurality of cams mechanically coupled to a central shaft are arranged to selectively actuate a plurality of electromechanical contacts. These contacts serve to connect the d-c electric traction motors on the car into particular configurations. For example, in one position the armature windings of several motors may be serially connected across a d-c power source so that each motor operates on some fraction of the total source voltage. In a second position the armature windings of pairs of motors on a truck may be connected across the d-c power source so that full source voltage is available to each pair of motors. The cam controller also controls contacts which selectively add or subtract series resistors from the armature current path of the motors in order to regulate armature current, or rather to maintain armature current at a constant level until the d-c motor characteristics force the current to decay below the regulated level. Thus the motive power produced by the d-c motors is controlled by controlling the resistance in series with the motors and the connection of the motors rather than directly regulating the d-c power source.
When a different type of motive power system is to be utilized in selected cars of a train in which other cars of the train employ cam controlled series wound d-c motors, it is desirable that the control systems of both types of motive power systems be capable of responding in an identical manner to the operator's command. For example, if an electronic controller capable of uniformly varying the power applied to a traction motor is to be used to propel one car of a train, it is desirable that the electronic controller be adapted to control its associated motors in such a manner that the torque or power developed by the electronically controlled motors is equivalent to or matches the power developed by each cam actuated, incremently variable resistor controlled motors. A similar problem arises if the traction motor on one car are of a different type such as separately excited or shunt-wound d-c motors.
Accordingly, it is an object of the present invention to provide a method and apparatus for matching the power developed by a d-c traction motor in an electronic power control system to that developed by a similar or different type of motor in a different type control system.